The invention relates to a device for synchronizing a clock pulse generator with a serial, electrically bivalent signal which can be received from an external medium and which comprises a number of successive bit cells of mutually nominally equal time length with at least one signal transition in each series of a predetermined, fixed number of directly successive bit cells, said device furthermore comprising:
(a) an oscillator with a control input for producing a control clock pulse train under the influence of a signal on the control input; PA1 (b) a clock extraction device for producing a read clock pulse train from a signal derived from said medium; PA1 (c) a phase comparator for receiving said control clock pulse train and said read clock pulse train and for supplying a phase difference signal; PA1 (d) a low-pass filter for receiving said phase difference signal and for forming therefrom a control signal for said control input. PA1 (a) during the synchronisation operation, the frequency/phase of the clock pulse generator must be quickly adaptable; PA1 (b) during the reading of the data information, the frequency should, in principle, be constant.
A device of this kind is known from U.S. Pat. No. 3,646,520. The known device utilizes a magnetic disc which comprises, in addition to data tracks, a separate clock track. Thus, only phase differences between the clock pulses on the disc and the data bits can occur. According to the invention, however, data information and clock information are stored together in a single track. In attractive data code, each bit cell comprises a signal transition at the beginning of the bit cell if the information has a first value; the other information value is then represented by either a signal transition in the centre of the bit cell, or by the absence of a signal transition. For the regenerating of the information, it is then necessary to generate a control clock pulse train which samples each bit cell in both havles in order to enable decoding of both information values.
The data bits are stored in groups on such generally mechanically driven, magnetic media (record carriers); such a group is also referred to as a "sector" and each sector is often separately addressable. There are tolerances in the driving of the medium. Moreover, the individual sectors are not stored together; in many cases notably reading and rewriting of only selected sectors takes place. As a result, a transition from one sector to the next sector is usually accompanied by a frequency jump as well as by a phase jump. For realizing a suitable read facility, each sector contains an amount of synchronisation information at its beginning, said information preceding the actual data information and being used for readjustment of the clock pulse generator as regards phase as well as frequency. In order to minimize the space required, the physical length of the synchronisation information must be limited and, therefore, the readjustment of the clock pulse generator may require only little time. Therefore, the clock pulse generator must be capable of quickly changing the phase as well as the frequency.
On the other hand, the content of the actual data information is arbitrary. Therein, the signal transitions do not occur exactly at the beginning or the centre of a bit cell: these transitions may be shifted up to 1/4 bit cell with respect to the nominal instant, it being possible for the shift to vary from one bit cell to another. The latter differences can be determined inter alia by the information content. These differences are such that, when averaged over a number of successive bit cells, they compensate for each other, i.e. the mean length of a bit cell is substantially constant. In order to enable correct reading of the information in spite of these differences, the sample frequency must be suitably constant: the adaptation to, for example, variations in the drive speed may only be effected slowly. The frequency of the clock pulse generator should then in principle be stable. Thus, a contradiction exists between the following two requirements: